This invention relates to a novel compound having a positive dielectric anisotropy which is useful as a component of liquid crystal materials, and a liquid crystal composition containing the same. 2. Description of the Related Art
The display mode of liquid crystal display elements utilizing optical anisotropy and dielectric anisotropy includes various modes such as twisted nematic (TN) type, dynamic scattering (DS) type, guest-host type, DAP type, etc., and the properties desired for liquid crystal compositions used vary depending on the respective modes. It has been required for any of the liquid crystal substances to be stable to moisture, air, heat, light, etc. and those exhibiting liquid crystal phases within as broad a temperature range as possible, around room temperature. At present, however, there is no single compound which satisfies such conditions so that several kinds of liquid crystal compounds have been mixed together and if necessary, non-liquid crystal compounds have been further mixed, for practical use.
Namely, it has generally been required for liquid crystal compositions used for display elements that in addition to the presence of liquid crystal phases within a broad temperature range including service temperatures, the viscosity is low; the operating threshold voltage is sufficiently small to be able to effect driving with a small power; the operating response rate is high; etc.
Some compounds which are similar in the structure to the compounds of the present invention are disclosed in some literature mentioned below.
Japanese patent application laid-open No. Sho 61-204168/1986 discloses a liquid crystal compound expressed by the formula ##STR2## wherein X represents F or Cl, Y represents ##STR3## and W represents cyano or an alkyl group of 2 to 15 carbon atoms.
Japanese patent application laid-open No. Sho 61-163864/1986 discloses a liquid crystal compound expressed by the formula ##STR4## wherein R represents an alkyl group or an alkoxy group of 4 to 8 carbon atoms.
Japanese patent application laid-open No. Sho 61-246167/1986 discloses a liquid crystal compound expressed by the formula ##STR5## wherein R represents an alkyl group or an alkoxy group of 1 to 10 carbon atoms and X represents ##STR6##
Further, D. Demus et al, "Flussige Kristalle in Tabellen" (1974), pp 254 (issued by VEB Deutscher Verlag fur Grundstoffindustrie ) discloses a liquid crystal compound expressed by the formula ##STR7## wherein R and R' each represent an alkyl group of 1 to 6 carbon atoms.